It has been recognized that one of the best ways of minimizing the environmental impact on the production of kraft pulp is to bleach the pulp without using chlorine or chlorine containing chemicals. Chemicals for this purpose include ozone, hydrogen peroxide, peracetic acid and like chemicals, and also oxygen for effecting delignification. One problem in treatment of pulp with these chemicals, however, is that transition metal ions (e.g. Mn, Fe, and Cu) if present in sufficient amount, affect the selectivity of the bleaching reactions, and thus can make the bleaching uneconomical. Conventional techniques for removing transition metals include evaporation, acid treatment, or acid treatment combined with chelation.
According to the present invention, a method is provided for removing transition metals from the treatment fluids in a cellulose pulp mill (typically a kraft mill) in a manner that renders the treated fluid reusable in the cellulose pulp mill. The treated fluid can be reused as needed in the fiberline, including in bleaching, or in the recovery area. The method is based upon the fact that zinc, cobalt, manganese, iron, copper, and the like precipitate out as solids if the pH is raised so that the liquid containing the dissolved metals is sufficiently alkaline. Typically if the pH is greater than 9, and preferably above 10.5, the metals precipitate out and can readily be filtered.
The precipitation can, according to the present invention, be aided by maintaining a certain concentration of suspended solids in the fluid being treated. These solids are added as lime, lime milk, lime mud, green liquor dregs, and/or slaker grits. The solids added will provide growth sites for the precipitating material and will also aid settling and filtration.
Other non-conventional methods of treating acidic bleach plant effluents include ultra-filtration and/or evaporation. However, these processes are much more energy and capital intensive than the process of the present invention.
According to the present invention, the source of alkali to raise the pH can be varied, but normally includes some type of lime, lime mud, lime milk, or slaker grits, and preferably some green or white liquor (clarified or unclarified, oxidized or unoxidized) or alkaline bleach plant effluents (as from a peroxide stage). A combination of alkaline liquids is preferred since it is desirable not only to obtain a high enough pH but at the same time maintain the calcium content at a minimum by maintaining a sufficient concentration of carbonate ions (at least about 0.01 moles per liter, and preferably about 0.05 moles per liter). Such a treatment can also have the benefits of removing oxalate, barium, calcium, sulfate, and possibly some of the organics, resulting in less scaling and reduced build up of sulfate in the bleach plant.
An additional source for carbonate ions can be carbon dioxide, e.g. from the flue gas from either the lime kiln, recovery boiler or a hog fuel boiler. Part of the liquor under treatment can be used to absorb carbon dioxide from the flue gas, or an external source of CO.sub.2. The carbon dioxide can be bubbled through the treated effluent, or can be scrubbed with effluent in a conventional scrubber.
Filtering can be accomplished by slurrying the filtrate with lime mud and passing it over a precoat filter, or by clarifying the filtrate and then using a polishing filter, or even ultra-filtration. If the source of alkaline medium used in the reactor contains particulates e.g., lime mud, slaker grits, etc., no filtering aid need be added prior to filtration. The particulates present in the treated effluent will act as a filtering aid.
According to one aspect of the present invention, a method of removing transition metals from treatment fluids in a cellulose pulp mill is provided which comprises the following continuous steps: (a) Treating digested pulp in an acidic or neutral stage to dissolve transition metals. (b) Washing the pulp after step (a) to produce a filtrate containing dissolved transition metals. (c) Adjusting the pH of at least a substantial part of the filtrate from step (b) so that it is greater than 9, while providing a filtrate carbonate content of at least about 0.01 mole/l., to cause dissolved transition metals in the filtrate to precipitate as solids in the pH adjusted filtrate. (d) Filtering the solids containing transition metals from the pH adjusted filtrate of step (c) to produce a greatly reduced transition metal content filtrate. And, (e) using the greatly reduced transition metal content filtrate elsewhere in the pulp mill.
Step (a) may be an ozone stage, or a chelation stage (such as an EDTA stage, an acid only stage, or a combination EDTA and acid stage). After step (b) the pulp may be further treated in at least one Z bleach stage at least one P bleach stage, and also preferably in at least one E stage (e.g. an E.sub.op) stage.
Step (c) is preferably practiced to adjust the pH so that it is greater than 10.5, and may be practiced by adding a material selected from the group consisting essentially of lime (e.g. reburned lime or slaked lime), lime milk, lime mud, slaker grits, alkaline bleach plant effluent, white liquor, green liquor, and mixtures thereof. The green and white liquor may be clarified or unclarified, oxidized or unoxidized. It is desirable to practice step (c) by adding carbonate ion so that the filtrate has a carbonate level of approximately 0.05 moles per liter, and the carbonate ion may be provided by using clarified or unclarified green or white liquor as part of the liquid used for pH adjustment. The carbonate ion may also be provided by means of carbon dioxide, for example from a mill combustion process.
Step (e) is typically practiced in a reactor. The temperature at which reaction takes place and the time of reaction are dependent upon the source of the acidic effluent. If the source is an ozone stage, the effluent arrives at the reactor at about 30.degree.-70.degree. C. and requires about 40-60 minutes reaction time. If the source is a chelating stage, the effluent arrives at the reactor at about 70.degree.-90.degree. C. and requires about 5-30 minutes reaction time. If it is desired to expedite the reaction, external heat can be provided (e.g. by indirect heating of the filtrate with steam).
Step (d) may be practiced by precoat drum filtering, precoat disc filtering, precoat planar filtering, or clarification followed by polishing filtering. Clarification may be practiced between steps (c) and (d) regardless of the method of filtration utilized. If an alkaline source containing particulate, e.g. lime mud, etc., is used, the particulate acts as a filter aid and a pre-coat filter may not be necessary.
While the greatly reduced transition metal content filtrate from step (e) may be used at a wide variety of locations within the pulp mill, one particularly good use is as wash liquid following an oxygen delignification stage.
According to another aspect of the present invention, a method of treating digested kraft pulp is provided which comprises the following steps: (a) Oxygen delignifying the kraft pulp. (b) Treating the oxygen delignified pulp in an acidic stage. (c) Washing the pulp after step (b) to produce a filtrate. (d) Adjusting the pH of at least a substantial part of the filtrate from step (b) so that it is about 10.5 or more. And, (e) filtering the pH adjusted filtrate.
According to still another aspect of the present invention there is provided a method of treating oxygen delignified medium consistency (e.g. about 6-18%) kraft pulp by the following steps: (a) Treating oxygen delignified medium consistency kraft pulp in an acidic stage to dissolve transition metals. (b) Washing the pulp after step (a) to produce a filtrate having dissolved transition metals. (c) Adjusting the pH of at least part of the filtrate from step (b) so that transitions metals in the filtrate precipitate as solids in the pH adjusted filtrate. And, (d) filtering the solids containing transition metals from the pH adjusted filtrate of step (c) to produce a greatly reduced transition metal content filtrate.
It is the primary object of the present invention to provide for the energy efficient removal of metal ions from cellulose pulp by acting upon liquids containing dissolved metal ions so that an effluent is produced, containing minimal metal ions, which can be used as needed in the pulp mill. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.